In recent years, electric vehicles (EVs), which involve less environmental burdens, have become popular and been adopted as replacements for gasoline-engine vehicles for an approach to conserve the global environment and to reduce the production of greenhouse gases. Such an electric vehicle includes a combination of a storage battery and a motor and basically needs to charge power from an external charger. For this reason, measures to reduce the consumption of battery power are required for the electric vehicles. In this respect, reducing the power consumption for heating (air conditioning), which requires a large amount of power consumption, is the most effective solution among the measures.
To this end, adopting a technique to directly conduct heat to a passenger (driver) via a steering wheel to be directly held by the passenger has begun. As an example of the technique, a configuration in which a heating wire serving as a heating element is embedded in the steering wheel is known.
More specifically, incorporating a heater into a steering wheel makes it possible to heat the steering wheel during a period even a little after the engine starts, i.e., even when each engine part is not yet heated enough. This configuration can reduce the opportunity to use heating and thus can reduce the power consumption while mitigating operational difficulty and/or discomfort caused by the cold steering device, for example, when a driver starts an automobile that has been parked outside in winter.
As an example of such a configuration, there is known a configuration in which a heater holding a heating wire on a sheet-shaped base member, for example, is wound around a cover portion having a linear shape or a three dimensional shape such as an annular shape. When a heater is wound around a cover portion that curves in an annular shape, for example, a wrinkle is formed at an inner circumferential position due to a difference in circumferential length between the inner circumferential and outer circumferential regions of the cover portion. Such a wrinkle appears as an observable irregularity in a state where the heater is covered by a skin portion, and thus deteriorates the touch-feeling of the steering wheel when the driver holds the steering wheel. In order to solve this problem, there is known a configuration in which notches are formed at two edges of the base member to cancel out the difference in circumferential length (see, e.g., Patent Literature (hereinafter, referred to as “PTL”) 1).
However, forming simple V-shaped or U-shaped slits (notches) at two edges of the base member alone is not sufficient, and a wrinkle or a rising portion of the base member is still formed at a position around the top of a notch when a heater is wound around the cover portion. For this reason, forming slits is not sufficient for preventing formation of wrinkles.
Moreover, there is known a configuration in which a heating wire is embedded in a sheet-shaped member by hot-pressing or the like (e.g., PTL 2).
In this configuration, the heater does not stretch well, and a wrinkle is easily formed in a state where the heater is wound around the cover member.
Furthermore, there is a known configuration in which a projection formed on a cover portion in a protruding manner is fitted into an opening formed in the base member for alignment when a sheet-shaped heater is wound around a linear cylindrical-shaped cover portion as a steering device of a motorcycle, for example (e.g., see PTL 3).
However, this configuration cannot be employed for automobile steering wheels. In the case of automobile steering wheels, the driver holds and grips the steering wheel during the operation of the automobile, so that it is likely that the driver feels the projection formed in a protruding manner in the cover member through a skin portion made of a soft material such as leather or urethane, for example, and it is thus difficult to make the driver comfortable with this configuration.
Meanwhile, there is another known configuration in which a large number of slits are formed in a base member of a sheet-shaped heater, so that the base member can be adhered to a curved cover portion while following the shape of the cover portion (e.g., see PTL 4).
In this configuration, an edge portion of the slit rises from the surface at an outer position of the curved part, thus forming a wrinkle.